Twin-screws were tried by the Admiralty some years earlier in the construction of the iron-cased floating batteries, but were driven in those vessels by one motion from the engines. The adoption of the twin-screw in their case enabled the Admiralty to build vessels that required only a moderately light draught of water, and carried, for their tonnage, an enormous weight of armament and armour, besides the weight of their engines; but the vessels had no increased powers of turning nor could they manœuvre rapidly under steam in any circumscribed space. The double independent screws overcame these drawbacks. A small vessel in the Clyde worked two screws also, with two rudders, the idea, as acknowledged by the adaptor, having been derived from the model exhibited in the Exhibition of 1851 by Mr. John Sturdee, master shipwright’s assistant at Portsmouth Dockyard. An unusual degree of interest attached to the trial of the steam-ship _Flora_ by reason of the fact that each of her twin-screws was to be operated by its own engine. In the light of future events it is worthy of note that up to this time it was thought that the twin-screw would be useful for smaller vessels and gunboats carrying six guns or less; whereas the _Flora_, as representative of ships capable of carrying large armaments of guns, with considerable engine-power, and a light draught of water, and with a power of manœuvring such as could not be possessed by a single-screw vessel, marked a step forward in the march of improvement which was destined to have far-reaching results, both in the Navy and the Mercantile Marine. So important was the trial deemed that the Admiralty sent special representatives to report thereon. The _Flora_ was an iron vessel, 150 feet long, 22¹⁄₂ feet beam, and 13 feet depth, and of 365 tons. She had two independent engines and screws, the latter being placed one under each quarter, and therefore in front of the rudder, in contrast to the prevailing system of placing a single screw right astern and behind the rudder. The cylinders of the two engines were 26 inches in diameter, with a stroke of 21 inches; and the propellers were each of 7 feet diameter with a pitch of 14¹⁄₂ feet. She had two tubular boilers working at 30 lb. pressure, and one high-pressure boiler working at 50 lb. pressure, the latter boiler being intended to be used for producing a steam blast in the chimney and to dry the steam from the two common boilers. The engines were of 120 horse-power collectively. She was rigged as a fore-and-aft schooner. The principal test to which the vessel was subjected tried her capabilities of being manœuvred. With the helm hard over and the engines going full speed ahead, the first circle was made in 3 minutes 14 seconds, the next in one second less time, and the third circle in 3 minutes 16 seconds, the diameter of the circle being about three lengths of the ship, but slightly diminished each time.